A common internal combustion engine is constructed so that up-down motion of a piston housed within a cylinder is transmitted to a crankshaft via a connecting rod and therefore the crankshaft rotates. Furthermore, generally, skirt portions are formed on an outer peripheral portion of the foregoing piston.
Incidentally, the piston operates in a wide range of lubrication condition among a boundary lubrication region, mixed lubrication region, and hydrodynamic lubrication region on the Stribeck curve, according to operation conditions of the internal combustion engine. Therefore, the lubrication situation differs between a thrust side and a counter-thrust side on the piston, and the lubrication situation also differs depending on sites on the piston.
Such differences in the lubrication situation occur within the foregoing skirt portions as well. That is, when the piston slides along an inner surface of the cylinder, plural portions that differ in surface pressure occur in each skirt portion. Of the plural portions, a portion where the surface pressure is high (a high surface pressure portion) has a state in which the oil film is thin; in particular, the oil film becomes thinnest when the piston reaches the top dead center. In connection with this, there arises a problem that seizure or abrasion is likely to occur.
Therefore, for the purpose of retaining lubricating oil on the skirt portions, a method in which a lubricating oil sump is formed on outer peripheral surfaces of the skirt portions by patterning that uses laser or a cutting process has been proposed. Furthermore, there also has been proposed a method in which the outer peripheral surface of the skirt portions is subjected to a surface treatment in order to improve anti-seizure property and sliding characteristic in the boundary lubrication region or the mixed lubrication region in which solid contact (contact between the skirt portions and the cylinder inner surface) occurs microscopically.
Still further, a method in which the patterning and the surface treatment, both mentioned above, are simultaneously performed on the outer peripheral surfaces of the skirt portions has also been proposed. As an inexpensive, simple and convenient example of this method there exists patterning performed on a resin coat. Further, there also has been proposed a method in which patterning is performed so that when the piston slides, lubricating oil will be guided from both end portions (low surface pressure portions) of the outer peripheral surface of the skirt portions to a central portion thereof (high surface pressure portion) (refer to the patent documents 1 and 2).